I'm no expert on the Japanese language, but when I watch Kenshin, I cannot help but notice the interesting terminology used within the show. Taking place at the beginning of the Meiji Restoration, a time of rapid change in Japan, many of the modern Japanese martial arts terms had not yet come into use. The creators of the anime apparently wanted to emphasize the changes that occurred during that time period by deliberately using older or more obscure terms rather than the more well-known modern terms, especially for describing various aspects of swordsmanship.
During the Edo period, when the Tokugawa Shogunate was in power, many samurai had become ronin, or wandering samurai, and a slow dilution of martial techniques occurred during this relatively war-free time. During the Meiji period and progressively afterwards, the use of martial arts as a means of actual combat was less emphasized in favor of using them as a method of physical and spiritual enlightenment.
In Rurouni Kenshin, Kenshin himself is the ultimate example of the changing times, himself the major player bringing about the Meiji Restoration. Although a god-like swordsman, he uses a sakaba-to (Nobuhiro Watsuki's made-up word for "reverse sword"), and to the surprise of the people he encounters, he uses it to protect people. Kenshin's style of Hiten-Mitsurugi-Ryu specializes in battou-jutsu, and he is known as the Battou-Sai (sword-drawing master). It is interesting that the term battou-jutsu is used at all, as more people are familiar with the equivalent modern term iaijutsu, the combative art of drawing the sword, or iaido, the modern, more spiritual, and less combative form of the art.
The man most famous specifically for "battou-jutsu," Hayashizaki Jinsuke Minamoto Shigenobu (1546-1621?), was a legendary swordsman who many consider to be the "father" of iaido. Historically, one could say that battou-jutsu sort of fits in between iaijutsu and iaido, just as the Japan of Kenshin's time was stuck between a time of war and a new era of peace. "Battou" literally means "drawing the sword," and it has been suggested by martial arts scholar Donn F. Draeger that Hayashizaki Jinsuke specifically developed techniques to strike defensively on the draw in response to an attack, despite the fact that many iaijutsu styles that preceded it also trained to attack on the draw (not necessarily for defense). As such, one can see why the battou-jutsu style is fitting for Kenshin, who does his best to refrain from fighting and killing.
The term "rurouni" itself is a made-up (probably) form of the more well-known term "ronin." The prefix "ru-" is probably the word for "wandering." Ronin (literally "wave-person or wave-people") were poor, wandering samurai having no masters to serve (as was the samurai's sole purpose in life). Trained in the arts of war and killing, during times of peace, they found themselves without reputable employment and often took to criminal activities. Ronin were often looked down upon, and the modern usage has similar connotations, as you might remember from Maison Ikkoku when Godai was called a ronin for not having been accepted into a university and therefore had to take a year off to reapply.
Kaoru's style of swordsmanship, Kamiya-Kasshin-Ryu, is an example of the evolution of the Japanese martial arts away from their combative roots toward a more spiritual form. As explained by Lillian Olsen, "Kasshin means to make the most of the spirit, and also signifies a style that lets people live, without killing." Even when fighting for real, Kaoru uses a bokuto (or bokken = wooden sword) rather than a naked blade.
The name "Kenshin" translates to the name of the third closing song (by J-pop superstar T.M. Revolution): "Heart of Sword". Ken = sword; shin = heart.
If you're interested in the history of Japanese martial arts, Kroch Library has an excellent selection of books on the subject. If you want to be like Yahiko and learn how to wield a shinai (four-piece bamboo sword), try the Cornell Kendo Club. I hear it's pretty fun.
[12/16/04 Update: For further articles by Lawrence Eng, see his anime page.]